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(11) | EP 1 482 551 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for forming a thin film of TiSiN, in particular for phase change memory devices |
| (57) The process for forming a film of TiSiN includes the following sequence of steps:
deposition of a TiN film at medium temperature, for example, 300-450°C, by thermal
decomposition of a metallorganic precursor, for example TDMAT (Tetrakis Dimethylamino
Titanium); exposition to a silicon releasing gas, such as silane (SiH4) and dichlorine-silane (SiH2Cl2) at 10-90 sccm -standard cube centimeters per minute- for a quite long time, for
example, longer than 10 s but less than 90 s, preferably about 40 s; exposition to
a H2/N2 plasma at 200-800 sccm, for 10-90 s, preferably about 40 s. |
[0001] The present invention relates to a process for forming a thin film of TiSiN, in particular for phase change memory devices.
[0002] As is known, phase change memory devices utilize a class of materials that have the unique property of being reversibly switchable from one phase to another with measurable distinct electrical properties associated with each phase. For example, these materials may change between an amorphous disordered phase and a crystalline, or polycrystalline, ordered phase. A material property that may change and provide a signature for each phase is the material resistivity, which is considerably different in the two states and varies of two or more orders of magnitude when the material transforms from the amorphous phase (more resistive) to the crystalline phase (more conductive) and vice versa.
[0003] Phase change may be obtained by locally increasing the temperature. Under 150°C, both phases are stable. Over 200°C, nucleation of crystallites is fast and if the material is kept to the crystallization temperature for a sufficient time, it changes phase and becomes crystalline. In order to change the phase back to the amorphous state, its temperature is brought over the melting point (about 600°C) and rapidly reduced.
[0004] From an electrical point of view, it is possible to reach both critical temperatures (crystallization and melting temperatures) by passing an electric current through a resistive electrode in contact or close proximity with the chalcogenic material and heating the material by Joule effect.
[0005] A chalcogenic element 1 based on the above is shown in Figure 1, and comprises a resistive electrode 2 and a chalcogenic region 3. The chalcogenic region 3 is generally in the crystalline state to allow good current flow. A portion of the chalcogenic region 3 is in direct contact with the resistive electrode 2 and forms a phase change portion 4.
[0006] By passing an electrical current of suitable value through the resistive electrode 2, it is possible to selectively heat the phase change portion 4 to the crystallization or melting temperatures and cause a phase change.
[0008] The present requirements of low-energy devices impose some constraints to the program current of the device. In order to meet such requirements, the resistive electrode should comprise a film with the following features:
a) very low thickness (5-50 nm);
b) high conformity, to ensure the electrical continuity of the electrode;
c) medium resistivity (i.e. 0.5-5 mΩ·cm);
d) temperature stability.
[0009] The presently available processes are not able to ensure all the above features. Indeed, deposited films with similar characteristics to the above are deposited using a PVD (Physical Vapor Deposition) or a CVD (Chemical Vapor Deposition) technique.
[0010] However, films deposited by PVD are scarcely conformal due to the specific deposition technique and thus cannot ensure the electrical continuity.
[0011] Currently available CVD process, in particular CVD processes developed to depose TiSiN layers, are all optimized to form low-resistivity films, in particular to form barrier layers in interconnects, and thus are not compatible with the requirements.
[0012] For example, C. Marcadal, M. Eizenberg, A. Yoon, L. Chen "Metallorganic Chemical Vapor Deposited TiN Barrier Enhancement with SiH4 Treatment" in Journal of The Electrochemical Society, 149 (1) C52-C58 (2002) discloses a commercial process using Metallorganic Chemical Vapor Deposition (MOCVD). In detail, this known process comprises three basic steps:
1. deposition of a TiN film at medium temperature (350-450°C) by thermal decomposition of a metallorganic precursor, namely TDMAT (Tetrakis Dimethylamino Titanium);
2. exposition to H2/N2 plasma;
3. exposition to a silane (SiH4) flow for 10 s.
[0014] The film obtained from the deposition in step 1 is very rich in carbon (30% atomic); carbon, present in the metallorganic precursor, causes the deposited film to be unstable when exposed to air (the film oxides and its resistivity increases in an uncontrollable way). The plasma treatment reduces the carbon content and at the same time thins the layer, so that the latter is less permeable to O2 (a thickness reduction is indeed observed following the plasma treatment). The subsequent silane treatment causes addition of silicon to the film; however, in the final layer, the Si concentration is quite small (about 4.4%) since the reactive sites have been dramatically reduced by the plasma treatment.
[0015] Since in the literature the resistivity of the end film is attributed to the presence of Si-N covalent bonds, it is clear that the known process can yield only a low resistivity film, since here the Ti-N metal bond is predominating.
[0016] Another known process (see e.g. "Low Resistance Copper Interconnects with MOCVD TiN(Si) Barrier for Sub-0.13 µm Applications", T. Suwwan de Felipe, et al., Novellus Systems and International SEMATECH) teaches the deposition of MOCVD TiNSi using TDEAT (Tetrakis (Diehtylamido) Titanium) and ammonia and soaking in SiH4 in-situ for forming a barrier for copper. Also here, the process is studied so as to reduce resistivity and thus is not suitable for forming a resistive layer, in particular for use in PCM devices.
[0017] The object of the invention is thus to provide a fabrication method affording higher resistivity levels for thin, high-conformal and thermally stable films of TiSiN.
[0018] According to the present invention, there is provided a process for forming a film of TiSiN, as defined in claim 1, and the material so obtained, as defined in claim 16.
[0019] For the understanding of the present invention, a preferred embodiment is now described, purely as a non-limitative example, with reference to the enclosed drawings, wherein:
- Figure 1 shows the basic structure of a known chalcogenic element; and
- Figure 2 shows the plot of the resistivity versus thickness for a film formed according to an embodiment of the invention and a film formed according to the prior art.
[0020] According to one embodiment of the invention, a film of TiSiN is obtained by the following sequence of steps:
1. deposition of a TiN film at medium temperature (300-450°C) by thermal decomposition of a metallorganic precursor, for example TDMAT (Tetrakis Dimethylamino Titanium);
2. exposition to a silicon releasing gas, such as silane (SiH4) or dichlorine-silane (SiH2Cl2) at 10-90 sccm (standard cube centimeters per minute) for a quite long time (for example, longer than 10 s but less than 90 s, preferably about 40 s);
3. exposition to a H2/N2 plasma at 200-800 sccm, for 10-90 s, preferably about 40 s.
[0021] All the steps are carried out in a same reaction chamber; in steps 2 and 3, the chuck is held at the same temperature as in step 1 (i.e. 300-450°C); in the alternative all the three steps are carried out keeping the chuck at the same temperature (in the range 300-450°C).
[0022] With the above process, the carbon present in the TiN film deposited in step 1 and occupying the reactive sites, is partly eliminated and replaced by Si that can thus be incorporated at a higher concentration than in the known process. In fact, the original film is exposed to the silicon releasing gas without previously eliminating the reactive sites and thus when still in a reactive condition; thus the original film is capable to absorb silicon in a greater extent than with the known process. Moreover the Si can also react with C forming Si-C bonds that together with Si-N bonds contribute to increase the resistivity of the original film; as a matter of fact Si-C and Si-N are covalent bonds typical of dielectric compounds. Therefore, at the end of step 2, the resulting film has a medium-level resistivity, as confirmed by tests carried out by the applicant. The subsequent plasma treatment eliminates the remaining carbon and completes the densification process, thus ensuring the required stability.
[0023] The film thus obtained is highly conformal, chemically stable and has constant resistivity, once exposed to air. If the film is used for an application requiring stability with temperature, as in case of the manufacture of PCM cells, in order to increase temperature stability, it is possible to carry out an RTP (Rapid Thermal Process) annealing in an inert environment (Ar or N2) at a medium temperature (600-800°C). Moreover it is possible to increase even more the resistivity of the original film above the level that can be obtained through the deposition conditions, by carrying out an additional or a subsequent RTP annealing in an oxidizing environment (O2/N2 at a medium temperature (500-800°C); the O2 percentage can vary from 0.01% to 10% depending on the annealing temperature. The final film resistivity, after this treatment, can vary in the range 1-10 mΩ·cm.
[0024] Figure 2 shows the results of tests carried out by the applicant for a layer of TiSiN deposited according to the prior art method and a layer of TiSiN deposited according the above embodiment of the invention, with exposition to silane after deposition of TiN. In both cases, the final layers derive from the superposition of thin layers of 5 nm, a complete cycle of plasma and silane treatment being performed for each thin layer.
[0025] In particular, in Figure 2, curve A represents the resistivity of the prior art layer and curve B represents the resistivity of the layer manufactured according to the invention. As may be seen, curve B shows a resistivity 2-3 times higher than curve A and ranging between 0.67 mΩ·cm for a 20 nm thick layer to 0.77 mΩ·cm for a 10 nm thick layer. Thus a TiSiN layer deposited with the method according to the invention has a resistivity higher than 0.6 mΩ·cm.
[0026] Furthermore, in the characterization of the material, the samples of TiSiN layer obtained with the above described method show a much higher content of Si with respect to samples obtained with the prior MOCVD method according the prior art (content of Si comprised between 4.5 at% and 21 at% for the process of the present invention compared to a maximum content of Si of about 4 at% for prior art processes).
1. A process for forming a film of TiSiN, comprising:
a) depositing a TiN film; and
b) exposing the TiN film to a silicon releasing gas to obtain a TiSiN film without previously exposing the TiN film to a carbon-reducing treatment.
2. A process according to claim 1, wherein the step a) comprises thermally decomposing
a metallorganic precursor, such as TDMAT (Tetrakis Dimethylamino Titanium).
3. A process according to claim 2, wherein step a) is carried out at 300-450°C.
4. A process according to any of claims 1 to 3, wherein silicon releasing gas is selected
between silane (SiH4) and dichlorine-silane (SiH2Cl2).
5. A process according to claim 4, wherein step b) comprises exposing the TiN film for
more than 10 s and less than 90 s.
6. A process according to claim 5, wherein step b) comprises exposing the TiN film for
about 40 s.
7. A process according to any of claims 1 to 6, comprising, after step b):
c) exposing the TiSiN film to a H2/N2 plasma.
8. A process according to claim 7, wherein the TiN film is exposed to plasma at 200-800
sccm.
9. A process according to claim 7 or 8, wherein the TiN film is exposed for 10-90 s,
preferably about 40 s.
10. A process according to any of claims 7 to 9, comprising, after step c):
d) annealing the TiSiN film in an inert gas environment.
11. A process according to claim 10, wherein step d) is carried out in an RTP equipment.
12. A process according to claim 10 or 11, wherein step d) is carried out at 600-800°C.
13. A process according to any of claims 7 to 12, comprising, after step c):
e) annealing the TiSiN film in an oxidizing gas environment.
14. A process according to claim 13, wherein step e) is carried out in an RTP equipment
15. A process according to claim 13 or 14, wherein step e) is carried out at 500-800°C.
16. A TiSiN film, characterized by a resistivity higher than 0.6 mΩ·cm.
17. A TiSiN film according to claim 16, having a resistivity in the range 1-10 mΩ·cm.
18. A TiSiN film according to claim 16 or 17, having a Si content comprised between 4.5
and 21 at%.